Field of the Invention
The present invention relates to a drive apparatus for a vehicle, and particularly relates to a drive apparatus of a vehicle provided with an engine, a motor generator and a transmission.
Description of the Background Art
Japanese Patent Laying-Open No. 2004-150507 discloses a hybrid vehicle provided with an engine, a motor generator and a transmission. In the hybrid vehicle, the transmission is disposed between a rotary shaft of the motor generator and a ring gear shaft that is mechanically connected to driving wheels and is configured to engage with or disengage from both shafts where necessary. Thereby, in the case where a slip occurs in the hybrid vehicle due to the idle running of the driving wheels, it is possible for the transmission to release the engagement between the rotary shaft of the motor generator and the ring gear shaft so as to detach the motor generator from the driving wheels. Accordingly, it is possible to prevent the motor generator from operating at an excessively large rotation speed due to the slip. As the slip is settled, the transmission engages the rotary shaft and the ring gear shaft at a small gear ratio. As a result, it is possible to prevent the vehicle form slipping again, which is very possible to occur immediately after the previous slip.
However, there is also a possibility that the excessive rotation of the motor generator occurs in the case where the driver is intending to continue the acceleration, which is not discussed by the prior art disclosed in Japanese Patent Laying-Open No. 2004-150507.
For the convenience of design, in a vehicle provided with an engine and a motor generator, it is common that the relationship between the upper limit rotation speed Nemax of the engine and the upper limit rotation speed Ngmax of the motor generator is designed to satisfy Nemax>Ngmax.
For such a vehicle, in the case where the rotation speed of the engine increases, in order to prevent the motor generator from rotating excessively, it is necessary to disengage the motor generator from the engine shaft before the rotation speed of the engine reaches the upper limit rotation speed Nemax. Therefore, a clutch (clutch K2 to be described below) is provided in the hybrid vehicle for disengaging the motor generator from the rotary shaft of the engine.
As the vehicle speed is increased in accordance with the intention of acceleration from the driver, the rotation speed of the rotary shaft of the engine will go beyond Ngmax and reach Nemax. Immediately before the rotation speed reaches Ngmax, the clutch is released, and thereby the motor generator is disengaged from the rotary shaft.
As the driver intends to continue the acceleration and thereby the vehicle speed is further increased, an upshift is performed by the transmission immediately before the rotation speed reaches Nemax. Due to such operation, the rotation speed of the rotary shaft becomes lower than Ngmax, and the clutch is engaged again. As the vehicle speed is further increased, the clutch is released again.
As described in the above, in the case where the driver intends to continue the acceleration, the clutch is engaged and disengaged repeatedly in response to the increase in the rotation speed of the rotary shaft during the acceleration and the decrease in the rotation speed of the rotary shaft at the upshift. Such hunting state causes a shock in the vehicle at the time of clutch operation and causes a torque to fluctuate in response to the engagement or the disengagement of the motor generator, which is not preferred.